1. Field of the Invention
This invention relates to the field of telecommunications. More specifically, the invention relates to communication across a transformer.
2. Background
An isolation barrier is generally used in applications in which it is desired to keep voltage potentials in one portion of a circuit separate from voltages in another portion, e.g., to prevent relatively excessive and/or harmful voltages from entering a relatively low voltage or voltage sensitive circuit. Such applications may include, for example, telephony, medical, industrial, and other similar applications.
For example, in a telephony application, it may be necessary to protect communication circuitry from high voltages on the telephone line by placing an isolation barrier between the communication circuitry and the telephone line. However, while it is desirable to prevent harmful voltages from crossing from one side of an isolation barrier to the other, it is also desirable to facilitate signal communication between circuits on both sides of the barrier. In telephony applications, the isolation requirement is generally imposed by some governmental requirement (e.g., FCC part 68 in the US).
The transformer is one of several types of electrical devices that may be used as an element of an isolation barrier. However, digital communication across a transformer in the prior art generally requires either a pulse transformer for each direction of communication, or time domain multiplexing of a pulse transformer (i.e., half-duplex communication). Prior art systems are incapable of full-duplex digital communication across a single transformer.
In practice, most prior art systems that are capable of bi-directional communication across a single isolation barrier element require a “flagman” to control the direction of communication. For example, if communication is occurring from the line-side to the host-side of the isolation barrier, a flag signals the sending side (line) driver to transmit while signaling the receiving side (host) driver to enter an inactive, high impedance state. For host-side to line-side communication, it is the line-side driver that is inactive.
Half-duplex communication in this manner reduces communication bandwidth as each direction of communication must wait its turn to use the one-way signal channel. However, the use of multiple transformers to achieve two-way communication is expensive in terms of cost and space. It would be much more desirable and advantageous to have a system that provides simultaneous bi-directional communication of signals across a single transformer.